Spray 8-inch square baking pan with vegetable oil cooking spray.
In a medium saucepan, melt the butter and saute the onion until tender, but not browned, for about 3 minutes. Remove the pan from the heat and add the muffin mix, egg, milk, sour cream, 1/2 cup of the cheese, the salt, and dill weed. Stir to combine. Pour into the prepared pan and top with the remaining 1/2 cup cheese. Bake for 30 minutes, until set and a toothpick inserted into the center of the cornbread comes out clean. Allow to cool slightly before cutting into squares.
These can also be made into muffins by simply spraying muffin tins with cooking spray and filling using an ice cream scoop. Bake time is 10 minutes using the muffin tins.